Mild hydrocracking is a hydrocracking process for converting heavy oil fraction to light products, controlled a conversion level within the range of 20-50% for the fraction boiling above 350.degree. C. (or above 370.degree. C.). The typical technological parameters are as follows:
pressure, MPa: 3.5-10.4
space velocity, hr.sup.-1 : 0.3-1.0
H.sub.2 /oil volume ratio, Nm.sup.3 /m.sup.3 : 300-1000
temperature, .degree.C.: 360-430
Comparing with high pressure hydrocracking, mild hydrocracking shows advantages in lower investment and operating cost, lower operating pressure, and especially lower (50%) hydrogen consumption. Currently, the mild hydrocracking technology is mostly used for increasing the yield of middle distillates (132.degree.-350.degree. C. fraction), while its tail oil (&gt;350.degree. or 370.degree. C. fraction) is used as the feedstock for FCC or as fuel oil, occasionally, is also used as the feedstock for steam cracking. Several European refineries have revamped their high pressure hydrocracking units for mild hydrocracking operation to provide feedstocks for steam cracking. The HC-K catalyst and the HC-16 hydrocracking catalyst are used in FPP process, developed by Union/Lummos in the United states. U.S. Pat. Nos. 4,738,767, 4,689,314, 4,600,703, and 4,396,500 disclose processes for preparing mild hydrocracking catalysts by impregnating .gamma.-Al.sub.2 O.sub.3 or SiO.sub.2 -Al.sub.2 O.sub.3 with the solution of Mo Ni-P or Mo-Si-P (Si stands for an organic silicon compound). Meanwhile, U.S. Pat. Nos. 4,600,498, 4,309,277, and 3,853,747 describe that the zeolites having weak acidity are used as acidic components in hydrocracking catalysts. Zeolite Y hydrothermally stabilized by Ni or Co is used for this purpose in U.S. Pat. No. 3,853,747. EP 0320247 describes the use of a hydrophobic zeolite (LZ-10) having a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of 3.5-4.0, surface area of 500-700 m.sup.2 /g, unit cell size of 24.25-24.35 .ANG., vapor adsorption capacity of less than 8% at 250.degree. C. and vapor partial pressure 4.6 mmHg: the zeolite and amorphous SiO.sub.2 -Al.sub.2 O.sub.3 are dispersed in Al.sub.2 O.sub.3 matrix to form catalyst support, then the support is impregnated with a Mo-Ni-P solution or W-Ni solution to prepare mild hydrocracking catalyst. A catalyst company in France files a patent (CN 85103645) in China which discloses a hydrocracking catalyst for producing middle distillate using a zeolite having medium acidity, a SiO.sub.2 /Al.sub.2 O.sub.3 mole ratio of 12-40, a crystallinity of, preferably, 60%, a corresponding surface area of 550 m.sup.2 /g, and 3-15% of its pore volume contributed by pores having a diameter in the range of 20-80 .ANG. (pore diameter of rest pores is less than 20 .ANG., no pores has a diameter larger than 80 .ANG.). The prior art mentioned above are mainly devoted to increase the production of middle distillates. However, the utilization of tail oil is not paid too much attention.